A developable latent image is formed in a silver halide emulsion layer of a radiographic element when it is imagewise exposed to X-radiation. Silver halide emulsions, however, more efficiently absorb and consequently are more responsive to longer (300 to 1500 nm) wavelength electromagnetic radiation than to X-radiation. Silver halide possesses native sensitivity to both the near ultraviolet and blue regions of the spectrum and can be sensitized readily to the green, red, and infrared portions of the electromagnetic spectrum.
Consequently it is an accepted practice to employ intensifying screens in combination with silver halide emulsion layers. An intensifying screen contains on a support a fluorescent phosphor layer that absorbs the X-radiation more efficiently than silver halide and emits to the adjacent silver halide emulsion layer longer wavelength electromagnetic radiation in an image pattern corresponding to that of the X-radiation received.
While the phosphor layer and emulsion layer can be integrated into one element, in most instances the adjacent silver halide emulsion layer is coated on a separate support to form a separate radiographic element. In this way, the intensifying screen, which is not permanently altered by exposure, can be reused. The most common arrangement for X-radiation exposure is to employ a dual coated radiographic element (an element with silver halide emulsion layers on opposite sides of a support), each emulsion layer being mounted adjacent a separate intensifying screen.
Phosphors employed in intensifying screens consist of particulate crystalline phosphors, typically oxides of a combination of metals. Often an oxide of one or two of the metals forms what is referred to as the phosphor host while oxides of one or more metals, often referred to as activators, are incorporated in the host in relatively low concentrations to change the hue and/or improve the efficiency of fluorescence. Lanthanides have been frequently employed as activators.
It has been recognized that the phosphors of highest absorption efficiencies are those in which the host compound contains at least one element from Period 6 of the Periodic Table of Elements. For example, barium sulfate, lanthanide oxyhalides and oxysulfides, yttrium tantalate, and calcium tungstate, are widely employed phosphor host compounds.
One family of phosphor host compounds that have shown promise in terms of performance, but have been little used are rare earth hafnates. L. H. Brixner, "Structural and Luminescent Properties of the Ln.sub.2 Hf.sub.2 O.sub.7 -type Rare Earth Hafnates", Mat. Res. Bull., Vol. 19, pp. 143-149, 1984, describes investigations of such phosphor host compounds. Ln is defined to include not only lanthanides, but also scandium and yttrium.
Another hafnium containing phosphor host compound that has been recognized to possess high efficiency in its absorption of X-radiation, but has enjoyed no practical use is optical grade hafnia--that is, hafnia that contains less than 3.times.10.sup.-4 mole zirconia. Kelsey U.S. Pat. No. 4,006,097, issued May 5, 1975, discloses to be useful in the absorption of X-radiation a phosphor satisfying the formula: EQU HfO.sub.2 :Yb
with Yb being present in a concentration of 5.times.10.sup.-3 to 1.times.10.sup.-1. Brixner, cited above, also investigated optical grade hafnia.
Lithium hafnate is a known compound. The following report investigations of lithium hafnate, but report no observations of luminescence:
R. Scholder, D. Rade, and H. Schwarz, Z. Anorg. Allg. Chem., 362, 149 (1968);
G. Dittrich and R. Hoppe, Z. Anorg. Allg. Chem., 371, 306 (1969);
J. L. Hodeau, M. Marezio, A. Santoro, and R. S. Roth, J. Solid State Chem., 45, 170 (1982); and
N. V. Porotnikov, V. V. Ganin, N. M. Gerardi, L. V. Golubeva, and K. I. Petrove., J. Russ. Inorg. Chem., 32, 764 (1987).
Chenot et al U.S. Pat. Nos. 4,068,128 and 4,112,194 disclose a variety of phosphors formed of varied ratios of phosphorus, hafnium, oxygen, and, optionally, zirconium. The various phosphor hosts produced by phosphorus in combination with hafnium are, of course, crystallographically dissimilar from hafnia host phosphors and offer no reliable indication of the effect of zirconium on the luminescence of monoclinic hafnia crystals.
Kroger et al U.S. Pat. No. 2,542,336 discloses a phosphor containing titanium as an activator and having a matrix composed of one or more of the oxides of zirconium, hafnium, thorium, germanium, or tin, to which may be added either acid or basic oxides or both. Suitable basic oxides are the oxides of sodium, potassium, rubidium, cesium, lithium, barium, calcium, strontium, magnesium, beryllium, and zinc. The acid oxides are those of the group SO.sub.3, B.sub.2 O.sub.3, P.sub.2 O.sub.3, and SiO.sub.2.